This application claims priority of the German patent application 101 25 469.5-51, which is incorporated by reference herein.
The invention concerns an apparatus for determining the light power level of a light beam.
The invention furthermore concerns a microscope.
Additionally the invention concerns a confocal scanning microscope.
The invention concerns as wellmethod for microscopy.
In order to measure the power level of a light beam, it is common practice to split a measuring beam out of the light beam using a beam splitter and first to determine the power level of the measured beam by means of a detector that generates an electrical signal proportional to the power level of the measured beam, in order then, with a knowledge of the splitting ratio of the beam splitter, to draw conclusions as to the power level of the light beam. The German Patent Application 197 02 753 A1 discloses an arrangement for monitoring the laser radiation coupled into a scanning head by means of a detection element onto which a portion of the coupled-in radiation is directed via a beam splitter.
Arrangements of the aforesaid kind achieve only limited accuracy and reproducibility. They are susceptible to external disturbances, in particular to vibrations and thermal influences.
In scanning microscopy, a specimen is illuminated with a light beam in order to observe the detected light, constituting reflected or fluorescent light, emitted by the specimen. The focus of an illuminating light beam is moved in a specimen plane by means of a controllable beam deflection device, generally by tilting two mirrors; the deflection axes are usually perpendicular to one another, so that one mirror deflects in the X direction and the other in the Y direction. Tilting of the mirrors is brought about, for example, by means of galvanometer positioning elements. The power level of the light coming from the specimen is measured as a function of the position of the scanning beam. The positioning elements are usually equipped with sensors to determine the present mirror position.
In confocal scanning microscopy specifically, a specimen is scanned in three dimensions with the focus of a light beam. A confocal scanning microscope generally comprises a light source, a focusing optical system with which the light of the source is focused onto an aperture stop (called the xe2x80x9cexcitation stopxe2x80x9d), a beam splitter, a beam deflection device for beam control, a microscope optical system, a detection stop, and the detectors for detecting the detected or fluorescent light. The illuminating light is coupled in via a beam splitter. The fluorescent or reflected light coming from the specimen arrives by way of the beam deflection device back at the beam splitter, passes through the latter, and is then focused onto the detection stop behind which the detectors are located. This detection arrangement is called a xe2x80x9cdescanxe2x80x9d arrangement. Detected light that does not derive directly from the focus region takes a different light path and does not pass through the detection stop, so that a point datum is obtained which results, by sequential scanning of the specimen, in a three-dimensional image. A three-dimensional image is usually achieved by acquiring image data in layers. Commercial scanning microscopes usually comprise a scanning module that is flange-mounted onto the stand of a conventional light microscope, the scanning microscope additionally containing all the aforesaid elements necessary for scanning a specimen.
A known method for compensating for or correcting fluctuations of the illuminating light power level is based on using a beam splitter to split a measured beam off from the illuminating light beam, and utilizing the ratio between the measured power levels of the measured beam and detected light beam for image generation or image calculation. This procedure is disclosed, for example, in the publication of G. J. Brakenhoff, Journal of Microscopy, Vol. 117, pt. 2, November 1979, pp. 233-242.
The aforementioned German Patent Application 197 02 753 A1 also discloses that in scanning microscopy, the formation of signal quotients or signal subtraction of a detected signal brings about noise reduction and can reduce intensity fluctuations. Because of the limited accuracy and reproducibility of the power level measurement already alluded to, however, the known microscopes can achieve only approximate compensation for fluctuations in the illuminating light.
It is therefore the object of the invention to propose an apparatus that allows the power level of a light beam to be determined in a largely accurate and reproducible manner.
The aforesaid object is achieved by an apparatus for determining the light power level of a light beam, the apparatus comprises: a beam splitter and a detector associated with the beam splitter, wherein the beam splitter splits measuring light out of the light beam and conveys it to the detector; and the ratio between the light power level of the light beam and the light power level of the measuring light measured at the detector is constant over time.
It is a further object of the invention to describe a microscope that makes possible reliable and improved elimination of and/or compensation for fluctuations in the illuminating light power level.
This object is achieved by a microscope which comprises: a light source which emits an illuminating light beam to illuminate a specimen, at least one detector for detecting a detection light proceeding from the specimen a device for determining the light power level of the illuminating light beam, having a beam splitter and a detector associated with the beam splitter, and the detector is arranged directly behind the beam splitter.
It is a further object of the invention to describe a confocal scanning microscope that makes possible reliable and improved elimination of and/or compensation for fluctuations in the illuminating light power level.
This object is achieved by a confocal scanning microscope comprises: a light source which emits an illuminating light beam to illuminate a specimen, at least one detector for detecting a detection light proceeding from the specimen a device for determining the light power level of the illuminating light beam, having a beam splitter and a detector associated with the beam splitter, and the detector is arranged directly behind the beam splitter.
An additional object of the invention is to describe a method for microscopy that makes possible efficient, reliable, and largely accurate compensation for fluctuations in the illuminating light power level.
This object is achieved by a method which is characterized by the following steps:
determining a light power level of an illuminating light beam with an apparatus that comprises a beam splitter and a detector associated with the beam splitter, in which context the beam splitter splits measured light out of the illuminating light beam and conveys it to the detector, and the ratio between the light power level of the illuminating light beam and the light power level of the measured light measured at the detector is constant over time;
determining a light power level of a detected light beam proceeding from a specimen; and
determining a corrected light power level by correlating the light power level of the illuminating light beam and the light power level of the detected light beam.
The invention has the advantage of making possible a reliable measurement of the light power level of a light beam. The invention furthermore has the advantage of reliable, interference-insensitive, and largely accurate compensation for light power level fluctuations, in particular fluctuations in the illuminating light for illuminating a specimen in microscopy and in scanning microscopy.
What has been recognized according to the present invention is that the known inaccuracies and unsatisfactory reproducibility of known apparatuses for measuring the light power level of a light beam are attributable, among other factors, to interferences within the measured light which result, in the event of even the slightest mechanical or thermal disturbance, in large fluctuations in the measured light power level. Accurate compensation for fluctuations in the illuminating light power level, or stabilization of the illuminating light power level in a microscope, is limited by these disadvantages.
In a preferred embodiment, the detector receives the measured light with reduced spatial and/or temporal coherence. For that purpose, a diffusing optical element such as a roughened glass plate or a milk-glass disk is provided in the beam path of the measured light. In a particularly preferred embodiment, the beam splitter comprises a substrate that has a diffusely scattering surface or that is at least partially made of milk glass.
A variant embodiment in which the detector is arranged directly behind the beam splitter is particularly advantageous. This variant is particularly insensitive to external influences, interference-resistant, and compact. Multiple reflections in the beam splitter often create several interference-capable partial beams, particularly at the coated and uncoated interfaces. The detector is therefore advantageously arranged in such a way that it is illuminated only by the primary split-off measured light, but not by other partial light beams. In a further advantageous embodiment, the entrance window of the detector itself, which preferably is coated in semi-reflective fashion, serves as the beam splitter.
In a preferred embodiment, the beam splitter is made of a substrate that comprises a semi-reflective coating. Preferably this is a metallic or dielectric coating. In another preferred embodiment, the coating is applied directly on the detector or on the entrance window of the detector housing. In a preferred variant embodiment, the beam splitter or coating is configured in such a way that the ratio between the light power level of the light beam and the light power level of the measured light measured at the detector is largely independent of the wavelength of the light beam.
In a further embodiment, the beam splitter (1) and the detector (11) are combined into one unit which comprises a housing.
Very particularly preferred is an embodiment having a beam splitter that generates one transmitted and one reflected partial beam, and exclusively the transmitted partial beam, constituting the measuring beam, strikes the detector. The reflected partial beam is directed, as the illuminating light beam, onto a specimen.
In a further embodiment, the microscope is a scanning microscope or a confocal scanning microscope that preferably comprises a processing unit that correlates the measured power level of the measuring light, in consideration of the splitting ratio of the beam splitter and other system parameters, with the power level of the detected light or of a portion thereof, for example the power level of a portion of the detected light from a specific spectral region. By determining a corrected light power level, fluctuations in the power level of the illuminating light are corrected. Very particularly advantageous is an embodiment in which offsets (caused, for example, by the dark current of detectors) can be determined prior to scanning and thus can be taken into account in the correlation. In a preferred embodiment, a processing unit comprising a programmable digital electronic system, for example a field programmable gate array (FPGA), is provided.
A semiconductor detector, such as a photodiode or an avalanche or PIN diode, a CCD chip, or a photodetector is preferably provided for determining the power level of the measuring light, since semiconductor detectors have a particularly small physical configuration. Photomultipliers or photomultiplier arrays can also be used.
Correlation of power level P of the detected light beam with power level M of the measuring light, taking into consideration offset P0 of the detector for measuring the power level of the detected light and offset M0 of the detector for measuring the power level of the measuring light, is preferably accomplished using the following formula:             M      korr        =                            P          -                      P            0                                    M          -                      M            0                              ⁢              (                              M            _                    -                      M            0                          )              ,
where {overscore (M)} is preferably the detected light power level averaged over an image or an image line, or over selectable scan points.
Taking the offsets, in particular, into consideration is particularly advantageous in this context, since if only the ratio of the detected light power level to the power level of the measurement is calculated, even offset components that are constant over time do not cancel one another out. Offsets can derive from the detectors, for example due to incorrect zeroing, or they can be caused by scattered or ambient light unintentionally reaching the detectors. In a particular embodiment of a scanning microscope, the offsets are determined automatically before an image is scanned. For that purpose, the illuminating light is interrupted and the signals proceeding from the detectors are measured and stored. Using a scaling step (which in the simplest case can comprise multiplication by a constant), the corrected light power level can be adapted to any desired scale.
The method according to the present invention comprises, in the first two steps, determining a light power level of an illuminating light beam with an apparatus that comprises a beam splitter and a detector associated with the beam splitter, in which context the beam splitter splits measuring light out of the illuminating light beam and conveys it to the detector, and the ratio between the light power level of the illuminating light beam and the light power level of the measuring light measuring at the detector is constant over time; and determining the light power level of a detected light beam proceeding from a specimen. Preferably both light power levels are obtained using detectors that generate electrical signals proportional to the respective light power level. In one embodiment, the signals are digitized and are correlated with one another in an FPGA unit or a PC; as a result, a corrected detected light power level is determined which can be used for image generation or image calculation. In another embodiment, correlation of the signals is performed in analog fashion.
In another variant embodiment, the light power levels of the illuminating light beam and the detected light beam are determined simultaneously.